Measured fraction of carboxyhaemoglobin depends on oxygen saturation of haemoglobin

The use of the OSM3 oximeter for measurement of the fraction of carboxyhaem oglobin (FCOHb) in blood allows for estimation of total circulating haemogl obin mass (Hb(tot)) by using the carbon monoxide rebreathing method. To ens ure high accuracy of Hb(tot) estimation, potential sources of analytical er rors should be identified and adjusted for. Based on observed differences i n results of measured FCOHb between simultaneously sampled, arterialized an d venous blood samples we investigated the influence of haemoglobin oxygen saturation (sO(2)) on results of measured FCOHb. Blood from nine healthy no n-smokers was tonometered with gas mixtures containing 94% N-2 or air and 6 % CO2. The resulting oxygenated and deoxygenated specimens were mixed in di fferent proportions to obtain varying sO(2) values in the same blood. sO(2) fractions of dyshaemoglobins, pO(2), pCO(2) and pH were measured at each s tep. FCOHb was significantly (p<0.001) higher in oxygenated (median, range: 0.6%, 0.4-0.9%) compared to deoxygenated (-0.2%, -0.5-0.0%) blood. Regress ion analysis identified the sO(2) as the most important factor explaining 8 6% of the variance in observed changes in FCOHb. The observed sO(2) effect has important implications on calibration procedure of OSM3, accuracy of me asured FCOHb, and FCOHb dependent calculations such as estimation of Hb(tot ) and related quantities. If the highest accuracy of FCOHb measurement is n eeded, an sO(2) effect on results of measured FCOHb has to be considered an d adjusted for.